The invention relates to interior lighting systems for a passenger transportation vehicle such as an airplane, a bus, or a ship and, in particular an LED reading light for an airplane, bus, or similar passenger transportation vehicle.
In passenger transportation vehicles, such as, for example, airplanes, buses, ships, or vehicles which run on rails (for example, railroad cars) seat lights or reading lights are used which are provided to an increasing extent with LED light means. So that the illumination is uniform, one aims to use LED light means which emit light at essentially equal intensity. In so doing, the set of problems consists in, among other things, the fact that the LED light means can only be produced with a non-negligible production-related tolerance. In the case of certain applications of an LED light means, e.g., in its use in a reading light for an air-plane, the LED light means is thus coded, which, for example, is done by a coding resistor which is read out by a power supply unit which actuates the LED light means and specifies with what current the LED light means must be operated in order to give off light with the desired light intensity.
For application in an airplane as an LED reading light, the LED light means are at present classified in one of a few classes (currently there are four classes) of LED light means and codes accordingly by the coding resistors. The power supply unit feeds a small test current into the LED light means when it is switched on in order to measure the voltage drop over the coding resistor. In so doing, the test current is chosen to be so small that the voltage drop over the coding resistor in any case remains under the forward voltage of the LED light means so that it (still) gives off no light. According to the value of the voltage drop, one of several (at present four) possible constant currents for supplying the LED reading light is then subsequently set.
As a consequence of the modernization of the LED manufacturing technologies and the use of the most varied semiconductor materials with the most varied dopings, to an increasing extent LED light means can be manufactured which can no longer be assigned to the classes of LED light means presently used in the aerospace industry. So one aims in the manufacture of modern LED light means to further reduce their power dissipation. Thus the forward voltages of the LEDs are being reduced ever further, which on the one hand means that the power supply currents for reaching the desired light intensity are becoming smaller but on the other hand also means that the voltage drop caused by the test current at the coding resistor no longer necessarily lies below the forward voltage. Both of these things make it more difficult to an increasing extent to meet the present specification requirements in effect in the civilian aerospace industry for LED light means for LED reading lights. The problem could, for example, be solved by the power supply units being adapted to the modern LED light means. This however is also not possible without problems since the changeover of the power supply units means a great technological effort which the manufacturers of, in particular, civilian airplanes do not wish to take on. Furthermore, the airlines demand that defective LED light means of reading lights can be exchanged for new LED light means without new power supply units also being required for the operation of these new LED light means.